This invention relates to a method for stabilizing halogen-containing polymers including poly(vinyl chloride) and to novel stabilized halogen-containing polymer compositions.
As is well known to those skilled-in-the-art, various halogen-containing organic polymers, particularly vinyl halide polymers which include homopolymers of vinyl chloride and copolymers of vinyl chloride with other monomers, may degrade when subjected to heat over an extended period of time as evidenced by darkening of the resin and by an increase in brittleness. These defects may render the resin unsuitable for many uses since the darkening produces an unsightly appearance, and the increased brittleness may cause mechanical failures. A wide variety of materials has heretofore been employed to stabilize halogen-containing polymers. Many of these additives have achieved some measure of success in stabilizing halogen-containing resins against the degradative action of heat, but there are many applications in which a greater degree of heat stability is desired than has heretofore been readily attainable.
It is particularly characteristic of prior art stabilizing systems that they may not fully prevent the deterioration of resins or polymers including poly(vinyl chloride) during the period when the resins and stabilizer composition may be maintained on the hot mill on which they are blended. During this period, which may be from 5 to 30 minutes or longer, the various ingredients including e.g. pigment, plasticizer, stabilizer, lubricant, etc. may be mixed with the resin and the mixture subjected to the influence of heat and pressure to form a substantially homogeneous mixture. During this period of severe heating (typically at 175.degree. C. or higher), the resin may deteriorate much more quickly than under normal processing or handling conditions. Thus the product coming from the Banbury Mill-Blender or extruder may be darker than is desirable.
Those skilled-in-the-art have heretofore attempted to eliminate this deterioration resulting from heat by addition of various materials including auxiliary heat stabilizers. It has been found, however, that the presence of these additional materials results in undesirable side effects which may unsatisfactorily modify the desired properties of the resin or polymer. Accordingly, it has heretofore not been possible to prepare resin compositions which may be stabilized in a totally satisfactory manner against the deteriorative effect of heat.
It is an object of this invention to provide a method for stabilizing halogen-containing organic polymer or resin compositions, particularly vinyl halide polymers, against heat deterioration. It is a further object of this invention to provide novel stabilized halogen-containing polymers. Other objects will be apparent to those skilled-in-the-art from inspection of the following description.